1. Field of the Invention
The present invention relates to an acceleration sensor for detecting acceleration, which is used for toys, automobiles, aircrafts, portable terminals and the like, and particularly to an acceleration sensor superior in impact-resistance.
2. Description of the Related Art
Acceleration sensor elements for use in the acceleration sensor, utilizing a change in physical quantity such as a piezo resistance effect and a change in electrostatic capacity, have been developed and commercialized. These acceleration sensor elements can be widely used in various fields, but recently, such small-sized acceleration sensors as can detect acceleration in multi-axial directions at one time with high sensitivity are demanded.
Since silicon single crystal is an ideal elastic body due to the extreme paucity of lattice defect and since a semiconductor process technology can be applied without large modification, much attention is paid to a piezo resistance effect type semiconductor acceleration sensor element in which thin elastic arms are provided at a silicon single crystal substrate, and the stress applied to the thin elastic arms is converted into an electric signal by strain gauges, for example, piezo-resistors, to be an output.
As a three-axis acceleration sensor element, an acceleration sensor element has been used, which comprises elastic arms each of a beam structure formed by a thin portion of a silicon single crystal substrate connecting a weight constituted by a thick portion of a silicon single crystal substrate in a center and a support frame in its periphery. A plurality of strain gauges are formed in each axial direction on the elastic arms. In order to sense a small acceleration with an enhanced sensitivity, the elastic arms are made long and/or thin, or the weight that works as a pendulum is made heavy. The acceleration sensor element that can detect a small acceleration has led to an excessive amplitude of the weight, when subjected to a large impact, and resulted to break the elastic arms. To avoid the break of the elastic arms even if a massive impact is applied, regulation plates have been installed above and below the acceleration sensor element to restrict amplitude of the weight within a certain range. The acceleration sensor element is housed in a protection case made of aluminum oxide or the like, and a bottom surface of a support frame of the acceleration sensor element is fixed to an inner bottom of the protection case with adhesive. Part of the inner bottom of the protection case facing a weight bottom of the acceleration sensor element usually works as a regulation plate on a bottom side.
Japanese Laid-Open Patents HEI 4-274005 (JP 04-274005 A), HEI 5-41148 (JP 05-41148 A) and HEI 8-233851 (JP 08-233851 A) disclose a structure in which, to control a gap at a predetermined value between regulation plates and a weight of acceleration sensor elements, small balls having a diameter of substantially the same distance as the gap are mixed into adhesive, and the adhesive mixed with small balls is used to bond the regulation plates to the acceleration sensor element. The gap can be maintained at a predetermined value because the gap between the regulation plates and the acceleration sensor element can be dictated by a diameter of small balls. The use of adhesive containing small balls thus enables the control of a gap between the regulation plates and the acceleration sensor element.
An upper regulation plate mounted on a weight and an inner bottom plate of a protection case used as a lower regulation plate can restrict movement of the weight within an appropriate range and avoid a damage of elastic arms. Among a number of acceleration sensors, however, a problem that ridges or corners of weight bottom surfaces made of silicon are chipped happens at a certain low frequency. When broken chips are held in gaps between weight top surfaces and the upper regulation plates or between the weight bottom surfaces and the inner bottom plates of the protection cases, normal movement of the weights are prevented, and acceleration to be measured cannot be measured in accuracy.
When a large acceleration component in a horizontal plane of an acceleration sensor element is applied to an acceleration sensor, a weight of the acceleration sensor element is largely swung in the acceleration direction, and edges of the weight bottom surface collide with an inner bottom plate of a protection case with a big impact. By the impact, the edges or corners of the weight bottom surface of the acceleration sensor may chip. When an acceleration sensor is freely dropped from a few meters high and collides with a wood on a floor or a ground, an impact caused by the collision is at least 4000 G to 5000 G. When such a large impact is applied to the acceleration sensor weight, edges or corners of the weight bottom surface may chip.
There may be an idea that edges and corners of weight bottom surfaces are chamfered or rounded to avoid the edges and corners of the weight bottom surfaces chipping even when the edges and corners of the weight bottom surfaces collide with the inner bottom plates of the protection cases and are applied a large impact. But, chamfering or rounding the edges or corners of the weight bottom surfaces is hard to machine and not practical. Alternatively, it may be thought that the protection cases are made of a soft metal or plastics, which may absorb an impact caused by the collision of the weights with the inner bottom plates of the protection cases. Metal and plastics have large thermal expansion coefficient that is much different from that of silicon of the acceleration sensor elements. The large difference in the thermal expansion coefficient may cause elastic arms to bend due to environmental temperature change and lead to offset in sensor output and noise. To decrease the offset in sensor output and the noise, the protection cases are made of ceramic or glass that has almost the same thermal expansion coefficient as silicon, of which the acceleration sensor element is made, or made of silicon like the acceleration sensor element.